Invalid merchant/shop! Error message

I have installed DIBS for WooCommerce plugin (version 2.5.2).
Then I wanted to test it and applied for test account in DIBS here: https://www.dibspayment.com/demo-signup In plugin settings I entered “DIBS Merchant ID” from test account.
All payment options appeared (DIBS, DIBS Invoice, DIBS MobilePay) but when placing an order I’m redirected to the page https://payment.architrade.com/paymentweb/start.action with an error message “Invalid merchant/shop!”.

Where can be the problem?